Climate and environmental change inferred from diatom communities in Lake Challa (Kenya-Tanzania)
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Lake Challa is a 5 km2, 97m deep freshwater crater lake located at 880m elevation on the southeast slope of Mount Kilimanjaro (Kenya-Tanzania). Information gained from a preliminary limnological survey of Lake Challa, presented here, will give insight into tropical limnological processes as well as aid in the paleoclimatological interpretation of a 25,000-yr diatom record. The survey, which included depth profiles of temperature, oxygen, pH and specific conductance, as well as epilithic and surface sediment diatom samples, revealed that Lake Challa is a meromictic lake which presently has at least 30 diatom species. A 22m sediment core, recovered in 2005, has well-preserved diatoms throughout its length. Diatom species assemblages were dominated by Nitzschia sp. 1 and Gomphocymbella sp. 1, for which the taxonomy is uncertain, and therefore general ecological characteristics were used in the data interpretation. An increase in Gomphocymbella sp. 1 may reflect an increase in the amount of benthic habitat and therefore lower lake levels. Diatom productivity may increase during periods of high precipitation and runoff which increases the total nutrient concentration, and high Nitzschia sp. 1 concentrations occur when input water has high Si:P ratios (warm, wet climate). Analysis of the diatom record suggests that Lake Challa experienced fluctuating intermediate lake levels from 25-21 thousand years before present (ka BP), low lake levels from 21-14.5 ka BP, sustained high lake levels from 14.5-11 ka BP, and high-intermediate lake levels from 11-5 ka BP but with short-term recessions at 10.6-11, 8.5 and 6.6 ka BP. After 5 ka BP, levels remained fairly stable, with a gradual decrease in lake level beginning at ~4 ka BP culminating at ~1.8 ka BP. Inferred climate changes from Lake Challa generally agree with other records in the area. Continuous sedimentary records from lakes in equatorial regions on which to base paleoclimatic reconstructions are rare. In conjunction with other biological and geochemical paleoproxy indicators, this high-resolution diatom record will help us better understand climate-driven environmental change in the vicinity of Mount Kilimanjaro, site of the only African ice-core record.